(edit: And to be clear, a good lawyer is 10% the written details in the contract and laws and regs and 90% understanding the relationship dynamics and your interests, being able to suggest a range of written structures and terms that capture tradeoffs, and being able to negotiate effectively. Ideally, a counselor.)
Some meta thoughts-
* there are all kinds of contracts and all kinds of relationships. Reasonable is in the eyes of the beholders, and depends on what they value (the business vs the person, etc). I have heard of much better (for the developer) terms, and much worse, than yours.
* from business perspective, the question ultimately is one of leverage. The question isn't how much you have contributed to the business- if your partner thinks that way then they value the relationship with you over the value to the business, and the terms you work out are dependent on how you value the relationship with them.
* a business is acquired for the future value it will bring to new owners. The question there is how much is your future contributions are worth to the business. If you are not needed or are easily replaceable and the product is turn key, then your value is zero. If your contributions are needed, then your value is very high and you should try to structure the future relationship accordingly.
Again, these are meta-thoughts only, the most important task is to try to find a good lawyer who is in your area and is familiar with these kinds of situations. Good luck.